Drill strings are known that are used in the fields of prospecting for and of working oil deposits, which strings are constituted by rods and possibly other tubular elements that are assembled end to end depending on drilling requirements. To make the borehole, the end of the drill string has a drilling tool which is set into rotation about its longitudinal axis with a load being applied to the drill string along said axis.
The diameter of the drilling tool is significantly greater than the ordinary diameter of the rods in the drill string, thereby leaving an annular space, referred to as a “drilling annulus”, around the drill string during drilling.
The drill string is made up of elements, and in particular of tubular drill rods assembled together end to end so that the assembled drill string presents an internal bore along its entire length. Drilling fluid such as a drilling mud is injected from the surface inside the drill string so that the drilling fluid flows down to the bottom end of the drill string, to the drilling tool, where it is injected into the bottom of the borehole. The drilling fluid serves to lubricate the drilling tool and to sweep the bottom of the hole so as to evacuate the debris produced by the drilling tool together with the drilling fluid which flows up in the drilling annulus from the bottom of the hole to the surface.
Because of the forces involved during drilling, the drill string becomes deformed inside the borehole, such that certain portions of the drill string can come into contact with and rub against the wall of the borehole. The level of friction torque can then become very high during drilling. In particular, for deflected boreholes, i.e. boreholes in which azimuth direction or inclination relative to the vertical can be varied during drilling, friction torque due to rotation of the drill string while drilling deflected boreholes with a large offset can reach very high levels that can damage the equipment used or make the drilling target unattainable. In order to avoid or limit contact between certain portions of the drill string and the wall of the borehole, in particular in portions of the borehole that slope little relative to the horizontal, and in order to avoid or limit the friction and wear that can result therefrom, the drill string is built up using drill string elements that have bearing zones of diameter greater than the nominal diameter of the rods in the drill string, and generally greater than the diameter of all other portions of the drill string elements. These maximum-diameter bearing zones come into contact with the wall of the borehole in the low portions thereof (i.e. portions situated below the practically horizontal axis of the hole) at zones that are spaced apart in the axial longitudinal direction of the drill string, so that isolated points of contact between the drill string and the borehole enable the friction torque on the drill string to be diminished.
Such maximum-diameter bearing zones can be provided, for example, on elements of the drill string such as tool joints, drill collars, or drill rods presenting profiles in a very wide variety of shapes. In particular, such bearing zones can be provided as described in French patent application FR-97/03207 in a portion of a drill rod adjacent to a zone for cleaning the borehole and for activating drilling fluid circulation, where the drill rod presents helical grooves of asymmetrical section. That enables those portions of the drill rod that include the drilling fluid activation grooves to be set into rotation inside the borehole without running the risk of coming into contact with the wall of the borehole.
In order to further improve the performance of bearing zones in terms of reducing friction at said bearing zones, French patent application FR-99/01391 proposes providing helical grooves in the outside surfaces of the bearing zones, the grooves being of cross-section that decreases in the axial direction and in the flow direction of the drilling fluid inside the drilling annulus. In this way, the fluid which flows in the axial direction inside the drilling annulus is channeled by the grooves of decreasing section in the zones where the string bears against the wall of the borehole, thereby producing flows in a radial direction around the outside surface of the bearing zone. This produces a hydrodynamic bearing effect at the bearing zone and decreases friction.
Nevertheless, machining bearing zones to obtain helical grooves of decreasing section is an operation that can be difficult and expensive. In addition, covering the bearing zones in a layer of wear-resistant material is made more difficult.
It is clearly preferable, for questions of cost and ease of manufacture, to make the bearing zones continuous and cylindrical in shape, being covered in a layer that provides protection against wear.